1. Field of the Invention
A varistor is used in medium- or high-voltage installations for measurement, protection or control puposes a cylindrical resistance body which is arranged between two parallel electrodes and is made from a sintered ceramic or a polymer which has been highly filled with sintered ceramic granules with a varistor character. The sintered ceramic or the sintered ceramic granules generally comprise(s) a zinc oxide which has been doped in a controlled manner with selected metals, such as Bi, Sb, Co and Mn.
The varistor is preferably used in surge arresters and has to be specified in such a way that it can conduct high-power current pulses which have been produced by lightening strikes or switching operations without being damaged. During the manufacturing process, such current pulses are applied to the electrodes of the varistor, in order to test their capacity to withstand high currents.
2. Discussion of Background
Methods producing such varistors are given in DE 34 05 834 C2 and EP 0,494,507 A1. In each of these methods, a cylindrical, ceramic resistance body based on zinc oxide is produced and an electrode is applied to each of the two parallel, planar end faces of the resistance body.
In the method described in DE 34 05 834 C2, circumferential steps are ground off the resistance body in the peripheral areas of both end faces. Then, the resistance body is provided with an insulating material which covers the circumferential face and the steps. After that, the end faces and some of the insulating material which has been applied to the steps are ground off. Finally, the metal electrodes are applied to the end faces in such a manner that they partly overlap the steps which have been filled with the insulating material but do not reach all the way to the edge of the end face. This method is extremely complex and, in addition, is susceptible to faults, since metal splashes may be formed in the peripheral area when the electrode material is applied, which splashes may lead to dielectric sparkovers when high-field current is applied. In addition, the incomplete coverage of the electrodes results in local overheating of the current density or the electric field in the resistance body, which overheating reduces the dielectric strength of a varistor which has been designed in this way.
In the method described in EP 0,494,507 A1, each of the electrodes is applied all the way to the edge of the end faces of the resistance body. Since, in a varistor of this type, each of the two electrodes extends over the entire end face of the resistance body, a homogenous electric field is formed inside the resistance body when a high current is conducted for a short time. This results in a uniform current density and therefore also in uniform heating of the varistor. Since the unprotected resistance body has sharp edges and points in the area of the outer boundaries of the end faces, and since the electrode material, which runs to as far as the outer boundaries, may pass into the circumferential surface of the resistance body, a ring made from a polymer with a high dielectric constant and with a high temperature stability is positioned on the circumferential surface of the resistance body. This ring ensures that the electric field is reduced in the circumferential surface, thus avoiding undesirable sparkovers. Again, such a method for producing varistors is extremely expensive and complex.
Accordingly, one object of the invention, as defined in the patent claims, is to provide a novel method of the type mentioned above for the rapid and economic production of a varistor. At the same time, a varistor produced using this method is to have both an excellent energy absorption capacity and a simple structure.
The method according to the invention is distinguished by the fact that it is suitable for series production and that it allows varistors with a high energy absorption capacity and a high capacity to withstand high currents to be manufactured quickly and economically.
The method according to the invention is distinguished by the following method steps:
A layer of electrode material is applied to each of the two end faces of the resistance body, which layer runs as far as the outer boundary of said end faces, and either a circular ring which is delimited by the outer boundary, runs to as far as the end face of the resistance body and has a width of from approx. 10 to approx. 500 xcexcm is removed from the layer, or the resistance body and, if appropriate, also the layer of electrode material is/are beveled at the outer boundary.
Unlike methods for producing varistors according to the prior art, in which very complicated and expensive processes are used to attempt to avoid the inevitable metallization flaws which occur when the electrode layers are applied, in the method according to the invention these flaws are removed subsequently.
On the one hand, the high energy absorption capacity and the high capacity to withstand high currents of the varistors produced using the method according to the invention stem from the fact that inhomogeneity in the electric field and in the current density in the varistor when a high-powder current pulse occurs are largely avoided as a result of the electrodes running to as close as possible to the outer boundary, which is designed as a sharp edge, of the end faces. Such inhomogeneity may be caused by metallized sharp-edge defects or by metal splashes which extend beyond the edge. Although a narrow electrode-free boundary or a bevel has a slight adverse effect on the ideal, homogenous state with electrodes running all the way to the edges, this measure does efficiently eliminate the considerable inhomogeneity (metallized edge defects which lead to failure).
On the other hand, the high energy absorption capacity and the high capacity to withstand high currents are also consequences of a suitable design of that surface of the varistor which is subjected to high dielectric loads between the two electrodes. In a first preferred embodiment of the varistor, this surface may comprise its cylindrical circumferential surface and two adjoining, circular sections, which are less than 500 xcexcm wide, of its end faces. In a preferred second embodiment, the surface contains bevels which run directly to the boundary of the electrodes and merge into the cylindrical circumferential surface of the varistor.